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Abstract:

An inverter filter is for a plurality of phases. The inverter filter
includes a node; a differential mode filter including for each of the
phases a first terminal, a second terminal, an inductor electrically
connected between the first terminal and the second terminal, and first
capacitor electrically connected between the inductor and the node. The
inverter filter also includes a third terminal structured to be grounded,
and a common mode filter. The common mode filter includes a resistor, and
a second capacitor electrically connected in series with the resistor
between the node and the third terminal.

Claims:

1. An inverter filter for a plurality of phases, said inverter filter
comprising: a node; a differential mode filter comprising for each of
said plurality of phases: a first terminal, a second terminal, an
inductor electrically connected between the first terminal and the second
terminal, and a first capacitor electrically connected between said
inductor and said node; a third terminal structured to be grounded; and a
common mode filter comprising: a resistor, and a second capacitor
electrically connected in series with the resistor between said node and
said third terminal.

2. The inverter filter of claim 1 wherein said differential mode filter
is selected from the group consisting of a sine filter and a dV/dt
filter; and wherein said first capacitor is electrically connected
between said second terminal and said node.

3. The inverter filter of claim 1 wherein said resistor is a first
resistor; and wherein said differential mode filter further comprises for
each of said plurality of phases a second resistor electrically connected
in parallel with said inductor and a third resistor electrically
connected in series with said first capacitor.

4. The inverter filter of claim 1 wherein said differential mode filter
is a sine filter; and wherein said first capacitor is electrically
connected between said second terminal and said node.

5. The inverter filter of claim 1 wherein said differential mode filter
is a dV/dt filter; and wherein said first capacitor is electrically
connected between said second terminal and said node.

6. The inverter filter of claim 1 wherein said plurality of phases is
three phases with or without a ground.

7. The inverter filter of claim 1 wherein said differential mode filter
is an active front end filter for a voltage source inverter; wherein said
inductor comprises a first inductor electrically connected in series with
a second inductor between said first terminal and said second terminal;
and wherein said first capacitor is electrically connected between said
node and another node between said first inductor and said second
inductor.

8. A system for a power source having a plurality of phases, said system
comprising: a voltage source inverter structured to interface said
plurality of phases to or from said power source, and to interface a
plurality of inputs from a generator or a plurality of outputs to a load;
and a filter comprising: a node, a differential mode filter comprising
for each of said plurality of phases: a first terminal electrically
connected to a corresponding one of said plurality of phases, a second
terminal electrically connected to a corresponding one of said plurality
of inputs or a corresponding one of said plurality of outputs, an
inductor electrically connected between the first terminal and the second
terminal, and a first capacitor electrically connected between said
inductor and said node, a third terminal structured to be grounded, and a
common mode filter comprising: a resistor, and a second capacitor
electrically connected in series with the resistor between said node and
said third terminal.

9. The system of claim 8 wherein said load is selected from the group
consisting of an induction motor, a permanent magnet motor, a synchronous
motor, a transformer, and a resistive load bank.

10. The system of claim 8 wherein said differential mode filter is a sine
filter; and wherein said first capacitor is electrically connected
between said second terminal and said node.

11. The system of claim 8 wherein said differential mode filter is a
dV/dt filter; and wherein said first capacitor is electrically connected
between said second terminal and said node.

12. The system of claim 8 wherein said system is a low voltage system.

13. The system of claim 8 wherein said system is a variable frequency
drive system.

14. The system of claim 8 wherein said system is a medium voltage
variable frequency drive system.

15. The system of claim 8 wherein said plurality of phases is three
phases without a ground.

16. The system of claim 8 wherein said plurality of phases is three
phases with a ground.

17. The system of claim 8 wherein said voltage source inverter comprises
an active front end; wherein said load is a motor; wherein said power
source is a power grid; and wherein said voltage source inverter is
further structured to enable reverse power flow from said motor to said
power grid.

18. The system of claim 8 wherein said voltage source inverter comprises
an active front end; wherein said power source is a power grid; and
wherein said voltage source inverter is further structured to enable
power flow from said generator to said power grid.

19. The system of claim 8 wherein said voltage source inverter comprises
a passive rectifier.

20. The system of claim 8 wherein said voltage source inverter comprises
an active rectifier.

21. The system of claim 8 wherein said differential mode filter is an
active front end filter for said voltage source inverter; wherein said
inductor comprises a first inductor electrically connected in series with
a second inductor between said first terminal and said second terminal;
and wherein said first capacitor is electrically connected between said
node and another node between said first inductor and said second
inductor.

Description:

BACKGROUND

[0001] 1. Field

[0002] The disclosed concept pertains generally to electrical filters and,
more particularly, to inverter filters having differential mode and
common mode filter characteristics. The disclosed concept also pertains
to systems including an inverter filter.

[0003] 2. Background Information

[0004] As shown in FIG. 1, a voltage source inverter 2 generates a
three-phase alternating current (AC) power output 4 using one of many
known pulse width modulated (PWM) control algorithms 5. Power is drawn
from a direct current (DC) power source 6 and energy is stored in a
capacitor bank 8. The DC power source 6 can be, for example and without
limitation, a 6-pulse rectifier buffered by AC reactance (not shown) of a
three-phase power source 10 and/or by a reactor (not shown) added on the
AC side 12 or DC side 14 of the DC power source 6. Relatively larger
systems can employ, for example, 12, 18, 24 or more pulses in conjunction
with a multi-pulse reactor (not shown) or transformer (not shown) to
remove harmonics drawn from the three-phase power source 10.

[0005] The voltage source inverter 2 powers a three-phase load 16 through
a three-phase output filter 18. Non-limiting examples of the three-phase
load 16 include an induction motor, a permanent magnet motor, a
synchronous motor, a transformer structured to change the voltage level
of a line-to-line output circuit, and a resistive load bank. Two known
non-limiting examples of inverter output filters include a sine filter 40
(shown in FIG. 3) and a dV/dt filter 42 (shown in FIG. 4), which only
filter the line-to-line power output 4 of the voltage source inverter 2.

[0006] Common mode problems arise in low voltage systems from an inverter
(e.g., without limitation, the voltage source inverter 2 of FIG. 1)
including transistors with parasitic capacitance coupled to ground G. The
example load 16 and its load power cables 19 also have parasitic
capacitance to ground G. This forms a common mode or zero sequence loop
of current flow that is largely orthogonal in terms of energy or power
transfer to the intended differential three-phase power circuit. In
medium voltage variable frequency drive systems, for example, some
topologies also have a ground electrical connection (e.g., a separate
ground electrical conductor is included with the three-phase AC power
electrical conductors), in order that the three-phase power output is
ground referenced.

[0007] Typically, when common mode (line-to-ground) problems arise in a
variable frequency drive system, a separate common mode filter (e.g.,
common mode filter 44 of FIG. 3 including three-phase common mode
inductor LCM and capacitors CCM) is added to remedy the
problem. However, the size and the cost of this separate common mode
filter 44 are approximately equal to the size and the cost of the
original line-to-line inverter output filter 18 (FIG. 1), the sine filter
40 (FIG. 3) or the dV/dt filter 42 (FIG. 4).

[0008] Referring to FIG. 2, in order to have power flow from a load 20
back to an AC power source 22 or from a generator 24 back to a power grid
26, a passive direct current (DC) power source, such as a passive
rectifier (e.g., the DC power source 6 of FIG. 1), is replaced by an
active rectifier (e.g., without limitation, a PWM rectifier 28). The
corresponding power electronics circuit employed for this purpose is
typically called an active front end (AFE) 30. The AFE 30, like a voltage
source inverter 2 (FIG. 1), generates a square wave and needs to be
filtered by an AFE filter 32 before electrical connection to the AC power
source 22 or power grid 26. The AFE filter 32 also has significant common
mode currents flowing from the active rectifier, such as the PWM
rectifier 28, to the AC power source 22 or power grid 26 and back to
ground 34.

[0009] FIG. 3 shows the known sine filter 40 and the known common mode
filter 44. These two separate components can be used in place of the
three-phase output filter 18 of FIG. 1. The separate common mode filter
44 can also be used with the AFE filter 32 of FIG. 2 to reduce common
mode currents. The capacitor(s) that comprise Cf can be individual (i.e.,
one per phase), or enclosed in a three-terminal can (not shown) in a wye
configuration (not shown).

[0010] FIG. 4 shows the known dV/dt filter 42, which, similar to the sine
filter 40 of FIG. 3, can be used upstream of the common mode filter 44 of
FIG. 3.

[0011] There is room for improvement in inverter filters.

[0012] There is further room for improvement in systems including an
inverter filter.

SUMMARY

[0013] These needs and others are met by embodiments of the disclosed
concept, which improve a differential mode inverter filter in order to
also provide a common mode (line-to-ground) filter function.

[0014] Preferably, low cost components are employed to form an electrical
filter to ground that significantly reduces common mode currents.

[0015] In accordance with one aspect of the disclosed concept, an inverter
filter for a plurality of phases comprises: a node; a differential mode
filter comprising for each of the plurality of phases: a first terminal,
a second terminal, an inductor electrically connected between the first
terminal and the second terminal, and a first capacitor electrically
connected between the inductor and the node; a third terminal structured
to be grounded; and a common mode filter comprising: a resistor, and a
second capacitor electrically connected in series with the resistor
between the node and the third terminal.

[0016] As another aspect of the disclosed concept, a system for a power
source having a plurality of phases comprises: a voltage source inverter
structured to interface the plurality of phases to or from the power
source, and to interface a plurality of inputs from a generator or a
plurality of outputs to a load; and a filter comprising: a node, a
differential mode filter comprising for each of the plurality of phases:
a first terminal electrically connected to a corresponding one of the
plurality of phases, a second terminal electrically connected to a
corresponding one of the plurality of inputs or a corresponding one of
the plurality of outputs, an inductor electrically connected between the
first terminal and the second terminal, and a first capacitor
electrically connected between the inductor and the node, a third
terminal structured to be grounded, and a common mode filter comprising:
a resistor, and a second capacitor electrically connected in series with
the resistor between the node and the third terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] A full understanding of the disclosed concept can be gained from
the following description of the preferred embodiments when read in
conjunction with the accompanying drawings in which:

[0018] FIG. 1 is a block diagram in schematic form of a voltage source
inverter, an output filter and a three-phase load.

[0019] FIG. 2 is a block diagram in schematic form of an active front end
filter, a voltage source inverter with active rectifier, and a motor or
generator.

[0020] FIG. 3 is a block diagram in schematic form of a sine filter and a
separate common mode filter.

[0021] FIG. 4 is a block diagram in schematic form of a dV/dt filter.

[0022] FIG. 5 is a block diagram in schematic form of a sine filter with
common mode filtering in accordance with an embodiment of the disclosed
concept.

[0023] FIG. 6 is a block diagram in schematic form of a dV/dt filter with
common mode filtering in accordance with another embodiment of the
disclosed concept.

[0024] FIG. 7 is a block diagram in schematic form of an active front end
filter with common mode filtering in accordance with another embodiment
of the disclosed concept.

[0025] FIGS. 8 and 9 are block diagrams in schematic form of systems in
accordance with other embodiments of the disclosed concept.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] As employed herein, the term "number" shall mean one or an integer
greater than one (i.e., a plurality). As employed herein, the term
"electrical conductor" shall mean a wire (e.g., solid; stranded;
insulated; non-insulated), a copper conductor, an aluminum conductor, a
suitable metal conductor, or other suitable material or object that
permits an electric current to flow easily.

[0027] As employed herein, the statement that two or more parts are
"connected" or "coupled" together shall mean that the parts are joined
together either directly or joined through one or more intermediate
parts. Further, as employed herein, the statement that two or more parts
are "attached" shall mean that the parts are joined together directly.

[0028] As employed herein, the term "node" shall mean an electrical
connection point for a number of electrical conductors, or an electrical
connection point for a number of electrical components.

[0029] As employed herein, the term "terminal" shall mean a node
structured to be electrically connected to a number of electrical
conductors or to an electrical apparatus.

[0030] As employed herein, the term "interface" shall mean to input and/or
to output.

[0031] As employed herein, the term "power source" shall mean a source of
electrical power (e.g., without limitation, an inverter; any suitable
alternating current power source), or a power grid.

[0032] As employed herein, the term "power grid" shall mean a network of
electrical conductors for distribution of electrical power, or a
distribution network for electrically connecting a plurality of sources
of power to a number of loads.

[0033] As employed herein, the term "electrically connected between" two
terminals shall mean that an electrical component, device or apparatus is
electrically connected to each of the two terminals, is electrically
connected in series with a number of other electrical components and is
intermediate the two terminals, or is electrically connected in series
with a number of other electrical components and is electrically
connected to one of the two terminals.

[0034] The disclosed concept is described in association with three-phase
electrical filters and systems, although the disclosed concept is
applicable to a wide range of electrical filters and systems for a
plurality of phases.

Example 1

[0035] FIG. 5 shows a three-phase sine filter 100 with a common mode
filtering function. For example, in this electrical filter 100, common
mode current flows through the parallel combination of three inductors
(Lf) 102,104,106. The common mode current is created by the voltages of
the phases (U,V,W) at the three-phase terminals 123,124,125 rising and
falling simultaneously and is an artifact of pulse width modulation of
voltage source inverter transistors (not shown, but see the voltage
source inverter 2 and PWM control algorithms 5 of FIG. 1). The electrical
filter 100 includes, as part of the conventional sine filter function,
three capacitors (Cf) 108,110,112. The electrical filter 100 further
includes, as part of a common mode filter function, the series
combination of a resistor (Rg) 114 and a capacitor (Cg) 116. The series
combination of the resistor Rg 114 and the capacitor Cg 116 is
electrically connected between a node 118, which is electrically
connected to each of the three capacitors (Cf) 108,110,112, and a ground
terminal 120. The electrical filter 100 provides filtered voltages for
the phases (U1,V1,W1) at the three-phase terminals 126,127,128 to a
three-phase load (not shown, but see the three-phase load 16 of FIG. 1).

[0036] The inverter filter 100 of FIG. 5 is for three example phases
U,V,W. This electrical filter 100 includes the node 118, the terminal 120
structured to be grounded, a common mode filter including the resistor
(Rg) 114 and the capacitor (Cg) 116, and a sine filter. The sine filter
includes, for each of the three example phases U,V,W, a first terminal
123,124,125, a second terminal 126,127,128, an inductor 102,104,106
electrically connected between the first terminal 123,124,125 and the
second terminal 126,127,128, respectively, and a capacitor 108,110,112
electrically connected between the respective inductor 102,104,106 and
the node 118.

Example 2

[0037] FIG. 6 shows a three-phase dV/dt filter 130 with a common mode
filtering function. The electrical filter 130 is somewhat similar to the
electrical filter 100 of FIG. 5, except that it also includes resistors
132,134,136 electrically connected in parallel with the respective
inductors 102,104,106, and resistors 138,140,142 electrically connected
in series with the respective capacitors 108,110,112. Like the electrical
filter 100, the electrical filter 130 includes the parallel combination
of three inductors (Lf) 102,104,106, and, as part of a common mode filter
function, the series combination of the resistor (Rg) 114 and the
capacitor (Cg) 116. The series combination of the resistor Rg 114 and the
capacitor Cg 116 is electrically connected between the node 118, which is
electrically connected to each of the three capacitors (Cf) 108,110,112,
and the ground terminal 120.

[0038] The inverter filter 130 of FIG. 6 is for three example phases
U,V,W. This electrical filter 130 includes the node 118, the terminal 120
structured to be grounded, a common mode filter including the resistor
(Rg) 114 and the capacitor (Cg) 116, and a dV/dt filter. The dV/dt filter
includes, for each of the three example phases U,V,W, a first terminal
143,144,145, a second terminal 146,147,148, an inductor 102,104,106
electrically connected between the first terminal 143,144,145 and the
second terminal 146,147,148, respectively, and a capacitor 108,110,112
electrically connected between the respective inductor 102,104,106 and
the node 118. The dV/dt filter also includes, for each of the three
example phases U,V,W, resistors 132,134,136 electrically connected in
parallel with the respective inductors 102,104,106, and resistors
138,140,142 electrically connected in series with the respective
capacitors 108,110,112.

Example 3

[0039] FIG. 7 shows a three-phase active front end filter 150 with a
common mode filtering function. For each phase (A,U, B,V and C,W), an
inductance is formed by the series combination of two inductors (Ls, Lf)
102',102'', 104',104'' and 106',106'', which series combination is
electrically connected between two terminals 161,158, 162,159 and
163,160, respectively. The three-phase terminals 158,159,160 are
electrically connected to or from a three-phase power source or power
grid (not shown, but see the three-phase power source 22 or power grid 26
of FIG. 2) and the other three-phase terminals 161,162,163 are
electrically connected from or to an active rectifier (not shown, but see
the PWM rectifier 28 of FIG. 2). Each of the capacitors (Cf)
108',110',112' is electrically connected between their common node 118
and another node 152,154,156 between the corresponding first inductor
(Lf) 102'',104'',106'' and the corresponding second inductor (Ls)
102',104',106', respectively. The capacitors (Cf) 108',110',112' and the
inductors (Ls) 102',104',106' and (Lf) 102'',104'',106'' form an active
front end filter for a voltage source inverter (not shown, but see the
voltage source inverter with the PWM rectifier 28 of FIG. 2). Like the
electrical filter 100 of FIG. 5, the electrical filter 150 includes, as
part of a common mode filter function, the series combination of the
resistor (Rg) 114 and the capacitor (Cg) 116. The series combination of
the resistor Rg 114 and the capacitor Cg 116 is electrically connected
between the node 118, which is electrically connected to each of the
three capacitors (Cf) 108',110',112', and the ground terminal 120.

[0040] The inverter filter 150 of FIG. 7 is for three example phases A,B,C
or U,V,W. This electrical filter 150 includes the node 118, the terminal
120 structured to be grounded, a common mode filter including the
resistor (Rg) 114 and the capacitor (Cg) 116, and an active front end
filter. The active front end filter 150 includes, for each of the three
example phases A,B,C or U,V,W, a first terminal 161,162,163, a second
terminal 158,159,160, an inductor (Ls) 102',104',106' electrically
connected between the first terminal 161,162,163 and the second terminal
158,159,160, respectively, and a capacitor 108',110',112' electrically
connected between the respective inductor (Lf) 102'',104'',106'' and the
node 118. The active front end filter 150 also includes, for each of the
three example phases A,B,C or U,V,W, a second inductor (Ls)
102',104',106' electrically connected in series with the first inductor
(Lf) 102'',104'',106'' between the first terminal 161,162,163 and the
second terminal 158,159,160.

Example 4

[0041] The inductors (Lf or Ls) in each of the example three-phase filters
100 (FIG. 5), 130 (FIG. 6) and 150 (FIG. 7) would normally be implemented
with a three-phase reactor because of size, weight and cost advantages.
For example, the common mode inductance (Lcm) is equal to Lf/3 when three
individual inductors are employed. However, one three-phase reactor (not
shown) having three iron core legs and a coil wound on each of the three
iron core legs is much cheaper and takes up even less space in an
electrical cabinet. This construction inherently has less common mode
inductance. Hence, Lcm<Lf/3 for a three-leg iron core reactor.

[0042] It is knowledge of how to raise Lcm, particularly in low voltage
three-phase reactors, that makes the disclosed concept practical. For
example, a 1.0 mH three-phase low voltage reactor would typically have a
common mode inductance of only about 50 μH (i.e., Lcm=0.05*Lf).

[0043] A typical three-phase inductor having three iron core legs and
three coils provides the same inductance as the inductance of three
individual inductors of any suitable construction type, but only for a
balanced three-phase sinusoidal circuit. Because an inverter has
ground-referenced voltage steps that are common to all three phases and
because any real world load has capacitance to ground, a common mode
circuit exists that is separate from the intended differential mode
circuit that controls, for example, the speed of the motor. The three-leg
iron core reactor construction has no theoretical common mode inductance
because the flux in the three iron core legs cancels. Of course, real
world inductors have leakage inductance or, in terms relating to
equations that govern coupled inductor design, the magnetic coupling
between real world coils is less than 1.0. In other words, every
three-phase set of inductors having inductance, Lf, has a common mode
inductance, Lcm, that is less than Lf/3 but greater than zero (i.e.,
0<Lcm<Lf/3).

[0044] The selection of resistor (Rg) 114 and capacitor (Cg) 116 is a
function of how much common mode filtering is desired, and the magnitude
of Lcm is a function of Lf. The easiest thing to achieve is dV/dt
reduction from each filter output phase-to-ground such that transient
peaks do not grow with relatively long distances to the load. This "dV/dt
reduction" removes the higher end of the spectrum that can, for example,
damage motor bearings, cause early failure of cable insulation, and, in
the case of an AFE filter on a wind generator system, stop AM radio
interference in the area around a wind farm.

Example 5

[0045] In the case of low voltage (i.e., a distribution voltage of less
than 1000 V) reactors, 0.05*Lf<Lcm<0.07*Lf.

Example 6

[0046] For medium voltage (i.e., a distribution voltage of about 1 kV to
about 10 kV) reactors, 0.1*Lf<Lcm<0.22*Lf.

Example 7

[0047] It will be appreciated that the phases U,V,W of FIGS. 5 and 6, and
the phases A,B,C of FIG. 7, can be phases with or without a ground. For
example, FIG. 5 shows (in phantom line drawing) a ground 122 with the
three phases U,V,W.

Example 8

[0048] FIG. 8 shows a system 200 including one of the filters 100,130 of
FIGS. 5 and 6. The system 200 is for a power source 202 having a
plurality of phases 204. The system 200 includes a voltage source
inverter 206 structured to interface the plurality of phases 204 from the
power source 202, and to interface a plurality of outputs to a load 210
through one of the filters 100,130.

[0049] FIG. 9 shows a system 200' including the filter 150 of FIG. 7. The
system 200' is for a power source 202' having a plurality of phases 204
and a motor or generator 208 having a plurality of phases 204'. The
system 200' includes a voltage source inverter (with active rectifier)
206' structured to interface the plurality of phases 204 to or from the
power source 202', and to interface a plurality of inputs from the motor
or generator 208 or a plurality of outputs to the motor 208.

[0050] It will be appreciated that the system 200' can also employ one of
the filters 100,130 of FIGS. 5 and 6 between the voltage source inverter
(with active rectifier) 206' and the motor 208.

Example 9

[0051] The load 210 can be selected from the group consisting of an
induction motor, a permanent magnet motor, a synchronous motor, a
transformer, and a resistive load bank.

[0053] The plurality of phases 204,204' can be three phases with or
without a ground (not shown, but see the ground 122 shown in phantom line
drawing in FIG. 5).

Example 12

[0054] The voltage source inverter 206' includes an active front end or
active rectifier (not shown, but see, for example, the PWM rectifier 28
of FIG. 2). If the motor or generator 208 is a motor, the power source
202' can be a power grid, and the active front end voltage source
inverter 206' can be further structured to enable reverse power flow
(e.g., during braking) from the motor to the power grid.

Example 13

[0055] The active front end voltage source inverter 206' can be structured
to enable power flow from the generator 208 to the power source 202',
which, in this example, is a power grid.

Example 14

[0056] The voltage source inverter 206 can include a passive rectifier
(not shown, but see the DC power source 6 of FIG. 1).

[0057] While specific embodiments of the disclosed concept have been
described in detail, it will be appreciated by those skilled in the art
that various modifications and alternatives to those details could be
developed in light of the overall teachings of the disclosure.
Accordingly, the particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of the disclosed
concept which is to be given the full breadth of the claims appended and
any and all equivalents thereof.